So, I was reviewing some old notes concerning microbial metabolism, and I came up on a problem. In some of my notes, I have it written that heterotrophs use pre-fixed carbon in the form of organics to meet their need of a carbon source because they cannot fix their own carbon. In other notes, however, I have it written that heterotrophs simply cannot use only carbon dioxode as their carbon source and therefore use organics as a supplement.

My core inquiry is this: It is rather obvious that heterotrophs use organics in order to obtain carbon, but is it also possible for them to make use of carbon dioxide in order to obtain carbon as a supplement to that obtained through organics?

_______________________________________________...Purely for the sake of science...

Yup, it's possible.The example can be seen in industrial fermentation of citric acid by Aspergillus niger. In order to produce citric acid, the CO2 produced from pyruvate decarboxylation is re-used again by combining it with another pyruvate to produce oxaloacetate (by the action of pyruvate carboxylase and PEP-carboxykinase enzymes).At this stage, we obtain 2 equivalent of oxaloacetate where both of them are combined with acetyl-CoA yielding 2 citrate where one of them are released out of the cell.In this example, we can see that the CO2 is re-used again. But it's not obtained directly from air CO2 like the plant does.

Hope this helps,Victor Apriel

Q: Why are chemists great for solving problems?
A: They have all the solutions.

So I guess a better way to ask my question is this: If a heterotroph can behave in a manner in which it obtains carbon from both organics and CO2, then what is the criteria that differentiates mixotrophs from heterotrophs?

_______________________________________________...Purely for the sake of science...

Well I can say it's still called as heterotrophs (chemoorgano heterotrophs), because in order to generate those CO2, they still need organic molecules as the sole electron donor.

And I can say that mixotrophs are organisms which do not belong to these 4 nutrition types:- Photolyto autotrophs- Photoorgano heterotrophs- Chemolyto autotrophs- Chemoorgano heterotrophs So if you found a microorganism which in example use a redox chemistry as a basis of energy yielding (chemo), using an inorganic electron donor like Fe2+ (lyto), but fixing organic compund also from an organic precursor (heterotroph), then I guess it's classified into mixotroph.

Q: Why are chemists great for solving problems?
A: They have all the solutions.